1. Field of the Invention
This invention relates to apparatus for reading information recorded on a magnetic storage device, such as magnetic tape. In particular, the invention relates to Faraday-effect magneto-optic transducer apparatus of a rotary form.
2. Description Relative To The Prior Art
A requirement for recording high-frequency signals on magnetic tape is that the tape move at a high speed relative to a magnetic record head, commonly called the head-to-tape scanning speed. In the prior art a high scanning speed is normally achieved by rotating a magnetic record head at a relatively high speed at an angle other than 0.degree. relative to the transporting of the tape. In this manner the magnetic head records information along a series of parallel slant tracks extending across the tape from edge to edge. By so moving the magnetic head, a high head-to-tape scanning speed is achieved without moving the tape itself at a high speed. Thus, high-frequency information can be recorded without consuming an excessive amount of tape.
When the recorded information is read from the tape, a playback head has to follow the recorded tracks exactly as they are put on the tape. This requires that the speed and direction of the playback head are controlled synchronously with tape playback speed and tape direction, respectively.
It is frequently desirable to play back the magnetic tape at a speed that is higher than the tape speed during recording, in order to quickly find a particular passage or one or more specific records. This is, however, difficult with slant-track equipment employing a conventional wire-wound magnetic read head in which, for the high frequency recording, the regular recording and playback speed may already be on the order of the upper limit of the capability of the equipment. For example, information at frequencies on the order of three megahertz can at modern packing densities be recorded at a head-to-tape speed of the order of about 250 centimeters per second. A tape playback speed on the order of fifteen times the tape recording speed would, for instance, be desirable. In this example, a playback speed on the order of 3750 centimeters per second would be required. From a mechanical point of view, this is well within the mechanical capabilities of, say, a rotary head form of tape transport.
However, an increase of normal operating speed for playback purposes results in a corresponding increase of playback frequency. For instance, when the high frequency band-edge of the magnetically recorded information is three megahertz, then playback at fifteen times normal head-to-tape speed results in an increase of the upper band-edge frequency to 45 megahertz.
The conventional wire-wound magnetic read heads are incapable of functioning satisfactorily at such elevated frequencies, particularly in view of eddy current losses in the core structure and self-resonant problems in the windings. This, in practice, severely impedes, if not prohibits, in many instances a searching of recorded information at a speed that is significantly higher than the normal operating speed, when such operating speed is already close to the existing capability of playback equipment, notably of a conventional magnetic playback head.
Magneto-optic transducers are known in the prior art for reading information recorded on magnetic tape. A particular advantage of a transducer of this type is its ability to resolve particularly small dimensions of tape magnerization. For example, U.S. Pat. No. 3,665,431 discloses a Faraday-effect magneto-optic transducer that may resolve a dimension of magnetization on the order of only 200 Angstrom units.
However, prior art magneto-optic transducers have been primarily of a stationary form. Accordingly, magnetic playback techniques employing a magneto-optic transducer have been limited generally to reading information recorded in a longitudinal format, i.e. information along a track parallel to the length of the tape, as only the tape is moving and the transducer is stationary. Longitudinal recording is generally limited to relatively low scanning speeds so as not to consume an excessive amount of tape. Thus, longitudinal recording is normally restricted to low-frequency applications, such as audio recording.
IBM Technical Disclosure Bulletin, Vol. 16, No. 11, published April 1974, discloses a magneto-optic device of rotary form, for read out of information recorded on a magnetic tape which is helically wrapped around a mandrel. A circumferential strip of magneto-optic material is coated on a surface of a ring prism, such surface being coplanar with the surface of the mandrel. Magnetization from the magnetic tape transfers to the magneto-optic material as the tape moves around the mandrel. A rotor having light-scanning and light-directing optics is mounted for rotation inside the mandrel. The light-scanning optics sweep a beam of polarized light in a circular direction along the strip of magneto-optic material, and the light-directing optics direct light reflected from the strip through an open end of the rotor.
In the magneto-optic apparatus disclosed in the IBM bulletin, light reflected from the magneto-optic material is modulated because of the Kerr effect, with the modulation corresponding to the magnetization transferred to the strip. A disadvantage of a Kerr-effect magneto-optic device, rotary or stationary form, is that linear resolution of the magnetic record is limited by the width of the light beam reflected off the magneto-optic material. A light beam having a cross-section on the order of microinches is the smallest beam possible. Accordingly, a magneto-optic transducer of the Kerr type limits the smallest possible dimension of magnetization that can be resolved, which thereby can restrict the high frequency response of the transducer.